Pole shoe for magnetic electron lens



Oct. 27, 1959 A. C- VAN DQRSTEN POLE SHOE FOR MAGNETIC ELECTRON LENS Filed Sept. 8. 19 53 I o6q INVENTOR ADRIANUS CORNELIS VAN DOR5TEN f M AGENT.

United States Patent Claims priority, application Netherlands September 9, 1952 16 Claims. (Cl. 250-495.)

The invention relates to an improvement in magnetic electron lenses used in electron microscopes and similar instruments.

e A reduction of the focal distance of a magneticelectron lens improves the quality of the lens, since the chromatic aberration is thus reduced. One of the advantages thereof is a reductionof the unsharpnessdue to fluctuations of the acceleration voltage of the electrons or of the energizing'current of. the lens, so that the stability requirements for the said electrical values may be less severe. The invention has for its object to provide a magnetic electron lens having a small focal distance and permits of obtaining further advantages.

The magnetic field between the poles of an electron lens penetrates into the bore of the pole shoes, particularly if the latter approach magnetic saturation. The field curve, i.e. the curve representing the magnetic field strength H in the axis of the lens as a function of the distance z along the axis, exhibits portions or spurs which extend in the direction of the axis and indicate the gradually weakening of the field strength. This is a harmful phenomenon, since or more non-magnetisable fillers.

Fatented Oct. 2?, 1959 in position by means of one In order to enhance the effect, twoor more of such cylinders may be arranged concentrically. 7

As an alternative, the desired effect may be obtained with the aid of an insertion piece, the outer diameter of which is equal to the diameter of the bore, if magnetically anisotropic material is to be used without making the entire pole shoe of this material.

A further method of obtaining the desired geometrical anisotropy consists in that that part of the pole shoe from which the effective flux emanates is wound by using a strip thickness and may be held of ferromagnetic ,materiaLthe turns being separated by it brings about .an increase in focal distance, so that the lens becomes weaker.

t The invention provides a simple and efficient means to reduce this disadvantage; it consists in that the pole shoes are rendered magnetically anisotropic in such a way that in the proximity of the bore the magnetisability in the direction of the axis is greater than that in a direction at right angles thereto.

This result may be obtained by means of a particular property of the material of the pole shoes (anisotropy of material) or by means of a particular construction of the pole shoes (geometrical anisotropy). As an alternative both means may be employed together.

Anisotropy of material may be obtained, for example, by making the pole shoes from an alloy containing 30 70% of iron and 70-30% of cobalt. Small percentages of other materials may be included, .such as manganese, vanadium-silicon or carbon. A typical alloy having very high magnetic properties consists of 47.5% cobalt, 49.5% iron and 3% other substances. If such an alloy is cooled in a magnetic field, the magnetic permeability in the direction of the magnetic field is considerably higher than in a direction at right angles thereto.

Geometrical anisotropy may be obtained by providing in the pole shoes a helical gap or one or more closed cylindrical gaps, surrounding the axis of the pole shoes. If desired, these gaps may be filled wholly or partly with non-magnetisable material. In general thegaps need not be deeper than the pole distance (the distance between the poles of the lens).

For this purpose the pole shoes maybe bored orelse a separate term-magnetic insertion piece, which does not form part of the magnetic circuit proper, may be inserted into the bore of the pole shoes. In its simplest shape this insertion piece is a tubule of term-magnetic material, the diameter of which is slightly smaller than the diameter of the bore. It need have only a comparatively small wall a layer of a non-magnetisable substance. The intermediatelayerisnot required if use is made of a magnetically anisotropic strip. For this purpose use may be made of the known iron-nickel tape used for making cores for loading-core coils, suclras described in United States Patent 2,147,791. In the direction at right angles to the surface and in the direction of length this tape is less magnetisable than in the transverse direction.

In order that the invention maybe readily carried into efitect, it will now be described with reference to the accompanying drawing.

Figs. 1 to S of this drawing show diagrammatically embodiments of pole shoes according to the invention.

Figs. 1 and 2 refer to a lens having gaps bored in the pole shoes. Fig. 1 is a sectional view in a plane through the axis of the lens. It shows a portion of the lens comprising the range in which the electrons are subjected to the action of the magnetic field. Fig. 2 shows the central portion of one of the pole shoes of the same lens, viewed in the direction of the axis.

Figs. 3 and 4 are also sectional views in a plane going through the axis of the lens. Fig. 3 shows a portion of a pole shoein which a closed gap is formed with the aid of an insertion cylinder and Fig. 4 shows a portion of a pole shoe having a helical groove, formed by means of a wound strip.

Fig. 5 shows the central portion of the pole shoe shown in Fig. 4, Viewed in the direction of the axis.

Figs. 6 and 7 showiield curves of magnetic electron lenses and serve to illustrate the effect obtained by means of the magnetic anisotropy.

The lens shown in Fig. 1 comprises two pole shoes 1 and 2. They are provided with bores 3 and 4 respectively, having a circular sectional area. The pole shoes have a relative position such that the axes of the bores 3 and 4 coincide. During the operation of the lens the electrons in the proximity of the axis 5 travel through the bore 3 to the bore 4. These electrons traverse an evacuated space, which may be bounded by the inner wall of the pole shoes or else by a separate tubule of non-ferromagnetic substance inserted into the bores of the pole shoes. By means of a field winding (not shownlor of a permanent magnetic system the pole shoes are magnetized in a manner such that opposite poles are formed at the pole surfaces 6 and 7 and a magnetic field is produced in the space or oriented in the direction of the axis 5. This field exerts.

a focusing effect on the electron paths.

In accordance with the invention the pole shoes are provided with cylindrical gaps 8 and 9. As long as mag netic saturation is not reached, these gaps exert little influence on the active field. In the case of a pole shoe without gaps magnetic saturation results in that the material bounding the bore is also saturated and a greater quantity of lines of force will emanate therefrom within the bores, so that the aforesaid spurs are produced. When the gaps :are provided, however, if the energization is not -too.st;rong, the gaps introduce reluctance to any radially-directed flux and thus prevent the surrounded material 12 from being saturated, so that this material 12 screens magnetically the space in the bore and thus suppresses the field in this space. On the contrary, the field strength in the proximity of the axis 5 in the field between the pole surfaces 6 and 7 increases. The width of the gaps 8, 9 may be 1. 5 mms. and the thickness of the wall 12, surrounded by it, may be 1 mm., the diameter of the bore being 6 mms. The depth of the gaps 8 and 9 may be 15 rnms., the pole spacing being 11 mms.

The same effect is obtained with. the pole shoes shown in Fig. 3 by providing the bore 3 with a widened portion 13, in which a separate cylinder 14 of ferromagnetic substance is arranged, the outer diameter of this cylinder being slightly smaller that the inner diameter of the widened portion 13. Consequently, an intermediate space 15 is left between the insertion tube14 and the wall surrounding it. The tube 14 is arranged in a manner such that its axis coincides with the axis 5 and is held in position by an intermediate piece 16, which partly fills out the space 15. This intermediate piece is made from a non-magnetisable substance, which is preferably conductive in order to avoid troublesome charges, for example, copper or aluminum. 7

As an alternative, two or more gaps of the kind shown in Fig. 1 may be provided to surround one another or else two or more cylinders of the kind shown in Fig. 3 may be arranged coaxially.

In the pole shoe shown in Fig. 4 the widened portion 17 has a larger diameter at the mouth of the bore 3 than the widened portion 13 in the case shown in Fig. 3. The widened poltion 17 comprises a ring 18, formed by a wound strip 19 of ferro-magnetic material, engaging a strip 20 of non-magnetisable material, for example, copper or aluminum. These strips may have thicknesses of 0.2 and 0.1 mm. respectively.

The depth of the gaps 8 and 9, the length of the tube 14 and the height of the ring 18 may be chosen comparatively at will. In general it may be stated that they should preferably not be smaller than the pole distance of the lens, i.e. the distance between the surfaces 6 and 7. It is not necessary that the edge of the cylinder 14 should lie in the surface 6.

A similar effect as that obtained by the constructions shown in Figs. 1 to 5 may be obtained by making the pole shoe portion shown from magnetically anisotropic material. which exhibits a higher magnetic permeability in the direction of the axis 5 (axial direction) than in a direction at right angles to this axis (radial direction). An embodiment of a pole shoe having this property is not shown, since the shape and the dimensions of such a pole shoe need not differ from those of the known types. The pole shoe must be made of a special kind of magnetisable material, for example, of an alloy mainly comprising a proximately equal percentages of iron and cobalt. In order to render a pole shoe made therefrom ma netically anisotropic, it is heated to, for example, 800 C., after which it is slowly cooled in a magnetic field. the flux of which is parallel to the axis 5 of the pole shoe.

It is a matter of course that in the lens shown in Fig. 1 the pole shoes 1 and 2 may be made of magnetically anisotropic material. The same applies to the ring 14, shown in Fig. 3 and to the additional rings which may be employed therein, but in general, when using anisotropic material, constructions as shown in Figs. 1, 2 and 3 need no longer be used in order to obtain geometrical anisotropy in addition to the anisotropy of the material.

In the construction shown in Fig. 4 the conditions are, however, different. In engineering, tape-shaped material having magnetically anisotropic properties is available and used, among other things, for manufacturing cores of loading core coils and is made of an alloy mainly comprising iron and nickel; it has been subjected to a treatment (glowing and cooling in a magnetic field) such that the magnetisability in the direction of length of the tape V (the direction of rolling) and in the direction at right angles to the surface is smaller than in the direction of width of the tape. Such a material is described in U.S. Patent No. 2,147,791. This magnetic anisotropy will, in

itself, be in general suflicient to obtain the effect aimed at by the invention, so that, when using this material, a second layer to space apart the successive turns is not required.

The inner end of a helical strip disturbs, of course, the radial symmetry of the lens, which adversely affects the quality. pressed by providing a closed cylinder of ferro-magnetic material coaxially in the ring formed by the wound strip.

In Fig. 6 the curve 21 illustrates the magnetic field strength H as a function of the distance z along the lens axis to the plane of symmetry 22 between the pole shoes of a lens of known construction in the condition of magnetic saturation. The field strength has a maximum at the point where the axis 5 intersects the plane 22. By improving the lens in accordance with the invention the curve is altered in a sense such that the flanks become steeper and the maximum rises. The curve assumes, for example, the shape of the curve 23. If the dimensions of the lens and also the energization (magneto-motive force) remain the same, the surface fHdl of the figure, enclosed between the field curve and the z-axis will not vary. However, the strength of the lens is proportional to fI-Pdz and this value increases according as the curve exhibits a higher maximum value of H, the surface remaining the same.

In certain cases it may be useful to apply the measure in accordance with the invention only to the pole shoe adjacent the image side or to provide a greater magnetic anisotropy for this pole shoe than of the pole shoe on the side of the object. In this case the field curve is not symmetrical as is shown in Fig. 6, but asymmetrical, as is shown in Fig. 7, by way of example. In this case the field strength decreases more rapidly on the image side than on the side of the object. It has been found that such avariation of the field strength is advantageous with respect to the spherical aberration of the lens. Conversely it may occur that it is desirable to have a steeper field curve on the side of the object, in order to keep the plane of the object outside the field, a spur being admitted on the image side in order to utilize the contrast effect produced in the case of a certain degree of spherical aberration owing to the formation of a background.

The invention is of importance not only for the manufacture of objective lenses having a short focal distance, but also for the manufacture of projection lenses. The magnification produced by such lenses is usually controlled by varying the strength of the energizing current. However, it may occur that an increased energization does not provide the expected higher magnification. It may even occur that the magnification decreases at an increase in energization. This phenomenon is due to the fact that in the case of a strongly energized lens, electron paths intersect the axis at a point lying within the operational field of the lens. Beyond this point the rays are thus subjected to a bending force in a sense opposite to the force exerted before this point. They are consequently bent towards the axis, which results in a reduction of the magnification.

Owing to the measure in accordance with the invention the area in which the magnetic field is operative is reduced, so that the risk of having the point of intersection of the cathode-rays and the axis within this area is diminished.

What is claimed is: t

l. A magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a gap therebetween, said pole shoes having axially-aligned bores each communicating with said gap for enabling a chargedparticle beam to pass axially through the bores and across the gap, the magnetic conductivity of one of the pole.

This phenomenon may be adequately sup-' shoes in the vicinity of its bore being greater in an axial direction than in directions perpendicular thereto.

2. A lens as set forth in claim 1 wherein both pole shoes exhibit the same magnetic properties.

3. A magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a gap therebetween, said pole shoes having axially-aligned bores each communicating with said gap for enabling a chargedparticle beam to pass axially through the bores and across the gap, and means associated with one of said pole shoes for increasing the radial reluctance thereof in the vicinity of its bore.

4. A magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a gap therebetween, said pole shoes having axially-aligned bores each communicating with said gap for enabling a chargedparticle beam to pass axially through the bores and across the gap, and means associated with one of the pole shoes for reducing the fringing field within its bore when the pole shoe approaches saturation.

5. A magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a gap therebetween, said pole shoes having axially-aligned bores each communicating with said gap for enabling a chargedparticle beam to pass axially through the bores and across the gap, at least one of said pole shoes being constituted of a material exhibiting a greater magnetic permeability in an axial direction than in directions at right angles thereto.

6. A lens as set forth in claim 5 wherein the pole shoe is constituted of anisotropic magnetic material having a preferential direction in the direction of the axis of the bore.

7. A magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a space therebetween, said pole shoes having axially-aligned bores each communicating with said space for enabling a chargedparticle beam to pass axially through the bores and across the gap, at least one of said pole shoes having a narrow gap of decreased magnetic permeability surrounding its bore.

8. A lens as set forth in claim 7 wherein the gap has an annular shape surrounding the bore and is susbtantially coaxial therewith.

9. A lens as set forth'in claim 7 wherein a non-magnetic member at least partially fills the gap in the pole shoe.

10. A lens as set forth in claim 7 wherein the narrow gap is cylindrical in form, surrounds the here, is coaxial therewith, and is slightly spaced by ferromagnetic material from the inside surface of the bore.

11. A magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a gap therebetween, said pole shoes having axially-aligned bores each communicating with said gap for enabling a chargedparticle beam to pass axially through the bores and across the gap, one of said pole shoes containing a cylindrical aperture aligned with the bore and extending only part Way therethrough, and an insert ferromagnetic cylinder coaxially arranged within said aperture and spaced from the adjacent wall of the bore by a non-magnetic medium.

12. A lens as set forth in claim 11 wherein a plurality of ferromagnetic insert cylinders, each spaced from the other, are coaxially arranged within the aperture of the pole shoe.

13. A lens as set forth in claim 11 wherein a ring constituted by a pair of wound strips is arranged within the aperture of the pole shoe, one of said strips being ferromagnetic and the other being non-ferromagnetic.

14. A magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a gap therebetween, said pole shoes having axially-aligned: bores each communicating with said gap for enabling a chargedparticle beam to pass axially through the bores and across the gap, said pole shoes in the vicinity of said gap being annular, at least the annular portions of one of said pole shoes being constituted by a wound strip of ferromagnetic material having a greater permeability in transverse direction than in the direction at right angles to the surface of the strip.

15. A lens as set forth in claim 14 wherein a ferromagnetic cylinder is coaxially arranged within the wound strip to eliminate disturbances of the field symmetry.

16. A lens as set forth in claim 1 wherein both pole shoes exhibit different magnetic properties.

References Cited in the file of this patent UNITED STATES PATENTS 1,121,859 Messiter Dec. 22, 1914 1,132,016 Jobke Mar. 16, 1915 2,053,162 Pfalzgrafi Sept. 1, 1936 2,370,627 Hillier Mar. 6, 1945 2,418,432 Smith Apr. 1, 1947 2,719,240 Walker Sept. 27, 1955 OTHER REFERENCES Analytical Experimental Physics, Chap. 24, Lemon, Ference, Stephenson, University of Chicago Press, 1956. 

